sounds like the shutter could use a good cleaning, I have a Deckel-Compur from the 20's that does this at 1-second and 1/2 second, usually the slower speeds go first. Ironically enough, it used to do this at 1/5th and 1/10th until I dropped it on concrete on accident now I got two more shutter speeds that are functional LOL

What shutter are you using? Don't throw it on the ground please it probably just needs to be opened up and cleaned with isopropyl alcohol but check if its safe for that shutter first since it can damage some very old shutters if it gets on the aperture blades and they are not made of metal